Fast magnetoacoustic wave trains with time-dependent drivers

Abstract

Context. Frequent observations of quasi-periodic rapidly-propagating wave trains in coronal structures have been made in the last decade. The dispersive evolution of fast magnetohydrodynamic waves propagating in coronal waveguides can provide a physical interpretation for many of these observations. Aims. Previous studies have considered the generation of fast wave trains by impulsive drivers which deposit energy instantaneously. The signatures of dispersively formed wave trains must depend on the temporal nature of the driver. We investigate the effect of varying the temporal width of the driving perturbation. Methods. 2D magnetohydrodynamic numerical simulations of impulsively generated wave trains in a guiding field-aligned density enhancement were performed with the novel addition of a time-dependant driver. Results. The final spatial and spectral signatures of the guided wave trains are found to depend strongly on the temporal duration of the initial perturbation. In particular, the wavelength (or frequency) of highest spectral amplitude is found to increase (decrease) with increasing temporal duration, whereas the spectral width decreases. Additionally, the efficiency of generation of fast wave trains is found to decrease strongly with increasing temporal width of the driver, with a cut-off at twice the internal Alfvén crossing time.